The present invention relates to valve metals and processes of preparing valve metals of acceptable quality for use in such applications as capacitors and the like.
There is always a desire in the industry to improve on the properties of valve metals, such as tantalum, particularly in the areas of capacitance and DC leakage achieved by valve metals when formed into capacitor anodes. The manner in which the various valve metal properties are improved upon include modifying various steps of the processes involved in making the valve metal, including purifying the valve metal.
There are general processes known to those skilled in the art for preparing valve metals. For instance, tantalum is obtained from ore and subsequently crushed into a powder. The tantalum is then separated from the crushed ore through the use of an acid solution and density separation of the acid solution containing the tantalum from the acid solution containing niobium and other impurities. The acid solution containing the tantalum is then crystallized into a salt and this tantalum containing salt is then reacted with pure sodium in order to reduce the tantalum containing salts to tantalum and to form salts with the non-tantalum containing elements. Water washing is then used to remove the salts and recover the tantalum which is subsequently subjected to one or more acid Teachings to remove chemical impurities. The tantalum is then dried resulting in what is known as a basic lot powder. Typically, this basic lot(s) is subjected to a heat treatment or thermal agglomeration step and then passivated to obtain a powder cake that is subsequently ground up into a powder. A deoxidation step using oxygen getters, such as magnesium, is then performed. After the deoxidation step, the tantalum powder is generally subjected to an acid wash and dried. The powder is then pressed into a pellet and sintered for subsequent processing by capacitor anode manufacturers.
There is an interest by those skilled in the art to nitride valve metals, such as tantalum, with the belief that such nitriding can decrease the DC leakage in capacitor anodes formed from such nitrided tantalum. The current nitriding techniques primarily involve the use of nitrogen containing compounds or nitrogen gas during the deoxidation stage. This method has several disadvantages including the lack of uniform distribution of the nitrogen in the valve metal.
Accordingly, there is a desire to improve on the methods of nitriding valve metals as well as improving on the resulting product.
A feature of the present invention is to provide processes to nitride valve metals, such as tantalum.
Another feature of the present invention is to provide a process which provides a more uniform distribution of the nitrogen throughout the valve metal.
A further feature of the present invention is to provide nitrided valve metals, such as tantalum.
An additional feature of the present invention is to provide a nitrided valve metal, such as tantalum, having high capacitance capability along with excellent flow properties and/or Scott density.
Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. The objectives and other advantages of the present invention will be realized and obtained by means of the elements and combinations particularly pointed out in the written description and appended claims.
To achieve these and other advantages, and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention relates to a process of preparing a nitrided valve metal which involves the step of nitriding a valve metal powder at a sufficient temperature and pressure prior to a deoxidation step. Preferably, the nitriding of the valve metal occurs during a heat treatment step but always prior to the deoxidation step.
The present invention further relates to nitrided valve metals wherein the nitrogen content is from about 1,500 ppm to about 4,000 ppm and is substantially and uniformly distributed throughout the valve metal. All ppm referenced herein are by weight.
The present invention also relates to a nitrided valve metal, such as tantalum, wherein at least a portion of the valve metal has pore sizes equal to or greater than 2 microns in size.
The present invention in addition relates to a nitrided tantalum having a capacitance of from about 40,000 CV/g to about 80,000 CV/g when formed into a capacitor anode and having a Scott Density of from about 25 to about 40 g/in3 and/or a flow of from about 70 to about 300 mg/s.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide a further explanation of the present invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate several embodiments of the present invention and together with the description serve to explain the principles of the present invention.